runtests: allow generating a binary sequence from hex

tests/FILEFORMAT.md

@@ -40,6 +40,19 @@ server's port number (in ASCII) followed by a space and the hexadecimal byte
 
     %b64[%HTTPPORT %9a]b64%
 
+## Hexadecimal decoding
+
+In the preprocess stage, a special instruction can be used to have runtests.pl
+generate a sequence of binary bytes.
+
+To insert a sequence of bytes from a hex encoded string, use this syntax:
+
+    %hex[ %XX-encoded data to decode ]hex%
+
+For example, to insert the binary octets 0, 1 and 255 into the test file:
+
+    %hex[ %00%01%FF ]hex%
+
 # Variables
 
 When the test is preprocessed, a range of "variables" in the test file will be

tests/runtests.pl

@@ -3304,6 +3304,13 @@ sub subBase64 {
         # put the result into there
         $$thing =~ s/%%B64%%/$enc/;
     }
+    # hex decode
+    if($$thing =~ s/%hex\[(.*)\]hex%/%%HEX%%/i) {
+        # decode %NN characters
+        my $d = $1;
+        $d =~ s/%([0-9A-Fa-f]{2})/chr(hex($1))/eg;
+        $$thing =~ s/%%HEX%%/$d/;
+    }
 }
 
 sub fixarray {